1. Field of the Invention
The present invention relates to a system for inspecting an EUV mask by using a charged particle beam, and more especially, to a system for grounding an EUV mask when the EUV mask is inspected by using a charged particle beam.
2. Description of the Prior Art
Optical inspection of a mask is based on a comparison of the light signals in the patterned regions relative to the non-patterned regions. A high contrast is necessary in order to achieve sufficient sensitivity for defect detection. The transmissive masks used in DUV (deep Ultra Violet) lithography can be inspected without difficulty since the contrast between the opaque regions and the clear regions is high at UV/DUV wavelengths. However, it is difficult to inspect the reflective masks, the EUV mask for example, used in EUV lithography since not only the contrast between the absorber region and the mirror region is low at UV/DUV wavelengths, but also wavelength of the UV/DUV is too lengthy to inspect EUV mask.
Optical inspection of a mask is based on a comparison of the light signals in the patterned regions relative to the non-patterned regions. A high contrast is necessary in order to achieve sufficient sensitivity for defect detection. The transmissive masks used in DUV (deep Ultra Violet) lithography can be inspected without difficulty since the contrast between the opaque regions and the clear regions is high at UV/DUV wavelengths. However, it is difficult to inspect the reflective masks, the EUV mask for example, used in EUV lithography since not only the contrast between the absorber region and the mirror region is low at UV/DUV wavelengths, but also wavelength of the UV/DUV is too lengthy to inspect EUV mask.
Now, a charged particle beam inspection system, an electron beam (E-beam) inspection tool, accordingly, is developed to inspect the EUV mask. However, accumulated charging on EUV mask will induce inspection issue while the EUV mask is inspected by the E-beam inspection tool. This issue will not happen to silicon wafer because silicon wafer can be grounded. Substrate of the EUV mask is dielectric, and cannot be grounded.
Furthermore, if the EUV mask on a moving stage is scanned continuously by the electron beam and charges are accumulated on the surface of the EUV mask without grounding, the contrast and intensity of the scanned images at different areas would not be consistent or equal during the inspection process. In other words, the electron beam scanning over different regions of the EUV mask would cause dwell times in order to make the images have better quality and consistent contrast and intensity during the inspection operation. The inspection speed and the throughput would be influenced greatly.